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(p40) "With the advent of high-compression motors, the tendency of a fuel to
spark-knock or detonate violently has attracted much attention. Knocking is
thought to be due to autoignition of part of the charge in front of the
flame. Mild knocking has little effect on the performance of the engine,
but severe knock may result in loss of power and damage to pistons and
bearings. Certain substances such as lead tetraethyl, iron pentacarbonyl,
etc. tend to prevent knocking. All hydrocarbons have different antiknock
properties, and certain of these, such as benzol and those in cracked
gasoline, are useful in suppressing knocking. Octane number is greatly
affected by altitude. The octane number may be about three units lower for
each 1,000ft elevation at elevations near sear level (growing to 7.5 per
1,000 ft at 12,000-ft elevation). Thus, if an octane number of 67.5 is
necessary at sea level, 50 is satisfactory at 6,000-ft elevation, and at
12,000-ft elevation even 20 is satisfactory."

The octane number required by cars of even the same make ranges by at least
7 to 12 Research octane units. Although additional power can be developed
by the use of high-octane fuels in suitable engines, the extra power of
mileage obtained by the use of high-octane-number fuels in current vehicles
is not impressive or conclusive. Under some conditions of operation a 10
per cent increase in mileage appears to be possible by increasing the octane
number from 68 to 83, but tests on some vehicles indicated no improvement or
even a decrease in mileage. High-octane fuels do provide smoother, more
satisfactory performance. "

I would just like to add to Tarun's post above that the elevation changes with a turbo motor will be far less pronounced that with a N/A car. Therefore if the existing car recommends 91 ovtane at sea-level, you will still have to use something very close to that at 6000ft elevation.

Not exactly. Did you get my last email explaning this? My original argument made no sense (regarding the wastegate). Anyways, here it is again...

<pre>
"Anyway, pretend this happens :

You are at sea level, and say the air
pressure is 15. Say you have a motor that
runs 10 pounds of boost (which is really 10
pounds above atm levels). Now the motor
see's 25 pounds.

Now we are at 1000MSL, and say the air
pressure has dropped to 10. The turbo will
still generate our 10 pounds of boost above
atm. Now the motor only see's 20 pounds,
but we've sacrificed a bit of power. We can
bump the boost to 15 pounds, and see the
same performance as at sea level (motor now
sees 25 pounds), thus negating our altitude
increase (and our benefit of running lower
octane fuel).